HR: 08:00h
AN: T51D-01 INVITED     [Abstracts]
TI: Double- and triple-dating of single detrital zircons with (U-Th)/He, fission-track, and U/Pb systems, and examples from modern and ancient sediments of the western U.S.
AU: * Reiners, P W
EM: peter.reiners@yale.edu
AF: Geology and Geophysics, Yale University, New Haven, CT 06517
AU: Campbell, I H
EM: Ian.Campbell@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
AU: Nicolescu, S
EM: stefan.nicolescu@yale.edu
AF: Geology and Geophysics, Yale University, New Haven, CT 06517
AU: Allen, C M
EM: charlotte.allen@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200
AU: Garver, J I
EM: garverj@union.edu
AF: Dept. of Geology, Union College, Schenectady, NY 12308
AU: Hourigan, J K
EM: jeremy.hourigan@yale.edu
AF: Geology and Geophysics, Yale University, New Haven, CT 06517
AU: Cowan, D S
EM: darrel@u.washington.edu
AF: Earth and Space Sciences, University of Washington, Seattle, WA 98195
AB: Measuring ages of multiple radioisotopic systems in single detrital crystals provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terranes. We developed methods for measuring U/Pb, fission-track (ZFT), and (U-Th)/He (ZHe) ages on the same single zircon crystals (He-Pb double-dating or He-FT-Pb triple-dating), providing both crystallization and cooling (eruption or exhumation) ages through one or two distinct closure temperatures ($\sim$$240\deg$C for ZFT and $180\deg$C for ZHe). U/Pb ages are measured by laser-ablation ICP-MS in exterior pits or polished surfaces of crystals, after which He and U-Th contents of bulk crystals are measured by laser heating and dissolution/isotope dilution. Multiple replicates of quickly-cooled volcanic zircons confirm accurate ages for each technique on single crystals. We demonstrate the utility of these methods with examples from: 1) Eocene paleofluvial deposits in Oregon that indicate extensive exhumation and volcanism at 45-50 Ma in the Idaho batholith and surroundings, 2) Miocene flysch of the Olympic Peninsula, which yields surprisingly few first-cycle volcanic zircons (He age = FT age = U/Pb age), instead indicating primary derivation from the interior Pacific northwest, where magmatism and exhumation appear to have been strongly decoupled, 3) modern sediment of the Mississippi river, illustrating the dominance of volcanic detritus younger than 100 Ma, from the western U.S., and 4) modern sediment from the Missouri river, illustrating roughly subequal contributions from 1.4-1.8-Ga Belt sources and $<%100-Ma volcanics, and a pronounced crystallization-age gap between 0.5-1.0 Ga. In two settings with active margin flysches (Kamchatka and Olympics), multi-method dating shows that zircon He ages were partially reset following burial (by $\sim$60-80% and $\sim$10%, respectively), despite the fact that the apatite fission-track (AFT) systems were only partially reset. We suggest that incomplete resetting of AFT systems in detrital apatite may often be caused by variable annealing kinetics, leading to erroneously low maximum burial temperature estimates.
DE: 8105 Continental margins and sedimentary basins
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting